Until recently, many scientists assumed that a steady increase in air temperature around the peninsula, the product of global warming, was the primary cause behind most of the ice loss. But new research looking at the western side of the peninsula suggests that this may not be the case after all. A study published Thursday in the journal Science suggests that warm ocean water may be the biggest driver of glacial retreat in that region — and it’s a problem that may not be getting enough attention.
Previous mapping projects around Antarctica “showed distinct patterns of glacier retreat around the peninsula from the earliest records to the latest, and we wanted to look into what was causing this,” said Alison Cook, a research fellow at England’s Durham University and lead author of the new paper, in an email to The Washington Post.
In a previous study, Cook and her colleagues had found that the pattern of glacial retreat on the peninsula did not match the behavior they would have expected if atmospheric warming were the main driver. And in recent years, the idea that ocean water can have a significant effect on glaciers has been gaining interest among glaciologists and climate scientists.
Many glaciers around Antarctica, particularly in West Antarctica, end at the edge of the sea. Often, these glaciers terminate in what’s known as an ice shelf — a ledge of ice that’s connected to the glacier but not grounded to the bedrock, so it essentially floats on top of the sea. These ice shelves are instrumental in keeping glaciers stable and holding back the flow of ice behind them. If they weaken or break, glaciers can quickly begin pouring ice into the water.
But because these ice shelves aren’t grounded, all the space below them is filled with ocean water, which presses against the front of the glacier right up to the point where the ice becomes connected to the bedrock. As climate change is causing not only the air around the world to warm but also the ocean, scientists are concerned that rising water temperatures may be helping to melt many glaciers from the bottom up. This process can cause ice shelves to thin and even break off, leading to rapid ice loss.
Cook and her colleagues decided to investigate whether these processes might be playing a major role on the western side of the Antarctic peninsula. This area has exhibited a clear pattern in ice loss over the past few decades, with glaciers in the south tending to retreat the most and glaciers in the north seeming to be more stable. Additionally, the researchers noted in the paper that although records of ocean temperatures around Antarctica tend to be sparse, there’s a sufficient store of data on the western Antarctic Peninsula to use for research purposes.
The researchers compared records of ocean temperatures and ice loss in the region between the years 1945 and 2009 — and they found a clear correlation. Nearly all the glaciers in the southern part of the peninsula, which flow out into regions with warm water — particularly, water that is warm at depths below about 100 yards or so — had experienced retreat during the study period. But glaciers farther north, in areas dominated by cooler waters, had experienced relative stability.
The relationship between ice loss and water temperature was further supported when the researchers examined changes in ocean temperatures and glacier behavior from the 1990s on. Records suggest that the ocean in this region warmed from the 1990s to the 2000s — and glacial retreat also began to accelerate in the late 1990s all along the coast, except for the coolest area in the northwest.
The warm water dominating the southern area comes from what’s known as circumpolar deep water, or CDW, a relatively warm water mass in the Southern Ocean. Research has suggested that the CDW has been growing warmer in recent decades, and also that its warm waters have been creeping higher onto the ice shelves in West Antarctica, potentially causing more melting.
Understanding these processes is critical to predicting the future of the Antarctic ice sheet, according to Richard Alley, a glaciologist at Pennsylvania State University.
“The ocean is warming, but much of the heat in the CDW that is implicated by the new paper has been in the ocean for a long time,” Alley, who was not an author of the new Science study, said by email. “Warming of the CDW may have contributed to the observed ice-sheet melting, but changes in ocean circulation bringing more CDW to the ice are clearly important, and probably most important.”
He added that these changes in the CDW have probably been influenced in part by the effects of anthropogenic climate change and associated changes in winds and water currents.
“Our results are key for making predictions of ice loss in response to ocean warming in this region,” Cook said, noting that it’s important to the influence of the ocean in future ice models. “The Antarctic Peninsula is one of the largest current contributors to sea-level rise, and the glaciers here are highly sensitive, so [they] are key indicators of how the ice will respond to future changes.”
Read more at Energy & Environment: